(1) Field of the Invention
The present invention relates to high pressure metal vapor discharge lamps, particularly metal halide discharge tubes, and a method for manufacturing the same.
(2) Description of the Prior Art
A translucent alumina withstands corrosive metal halides, and is used for tubular bodies of high pressure metal vapor discharge lamps, particularly for tubular bodies of metal halide lamps in which a metal halide is sealingly placed. On the other hand, alumina or cermet is used for end plates adapted to fit electrode support members at end portions of the tubular body. When the discharge tube is to be produced by assembling these parts together, it is a common practice to bond the end plates to the opposite ends of the tubular body made of the transluscent alumina with a frit (for instance, U.S. Pat. Nos. 3,885,184 and 4,001,625). The tubular body has been made translucent through preliminary firing.
However, the use temperature of the discharge tube produced by such a method cannot be sufficiently raised since there is a fear that the frit is corroded with the metal halide, so that the discharge efficacy must unfavorably be suppressed to a level far lower than a theoretically calculated value. In addition, even if such is taken into consideration, only a relatively short durable life can be attained. Therefore, a method which allows easy production of discharge tubes for metal halide lamps which possess high discharge efficacy and long durable life has been demanded.